The present invention relates to servomechanisms and control systems, and more particularly, to a novel servomechanism control system and method featuring automatic servo loop parameter adjustment during movement, according to speed and position tracking error information.
In the past few years, motor drive technology has switched from analog to digital control, and this change in the design approach achieves better noise immunity, and allows implementation of sophisticated drives for AC and DC motors at lower cost. Closed-loop servomechanism control systems applied to electrical motors are arranged as a feedback loop which comprise certain basic elements, including an error detector, which generates an error signal representing the difference between the command signal and the controlled output, and a controller, which amplifies the error signal. The error detection is based on use of a feedback sensor which is used to measure the state of the system, and feed the value back to the control system. For example, in the case of electrical motor control, a position feedback device mounted on the motor shaft provides position information. Another approach to motor control relates to control of the electrical current. Many other control systems can be designed using this approach.
Transient analysis of servomechanism control systems provides a definition of certain basic concepts regarding system performance, and PID (proportional-integral-derivative) control is based on these concepts. The typical issues affecting design of these systems relate to the speed of response to disturbances in a closed-loop feedback system, the amount of overshoot which can be tolerated, and the steady state error which obtains after the system has reached equilibrium.
In any servomechanism control system design, the introduction of PID control presents constraints on the achievement of good speed of response, while minimizing offshoot, and avoiding oscillations which cause system instability. Thus, achieving a well-designed servomechanism control system becomes a complicated goal when considering the need to simultaneously achieve accurate, automatic, continous and instantaneous performance. The tightness of control is the goal, and the methods of achieving it include raising the amplitude of the response to a given error, which achieves more rapid response, but with this comes the oscillation problem.
Therefore, it would be desirable to provide a method which improves the tightness of control for servomechanisms, without unduly complicating the apparatus and its operation.
Accordingly, it is a principal object of the present invention to overcome prior art problems associated with PID control of servomechanisms and the speed of response, and provide an improved method for controlling a servomechanism.
In accordance with the present invention, there is provided an improved method of controlling a servomechanism, said method comprising the steps of:
generating a command signal representing a desired physical quantity;
actuating a driven element to output said physical quantity in accordance with said command signal;
measuring said output physical quantity and comparing it with said command signal;
generating an error signal representing the difference between said command signal and said meaured output; and
providing said error signal in a feedback loop having a variable gain to actuate said driven element in a fashion so as to reduce said error signal to a zero value,
wherein said variable gain is provided in accordance with a function related to the absolute value of said error signal and the absolute value of the velocity of said command signal.
The inventive method is applicable to nearly all types of servomechanism control systems. In general application, it is useful to control the state of a system, using an actuator, which provides a means of changing the state of the system, and a feedback sensor, which provides a means to measure the state of the system, to provide a feedback value to the control system. Examples of these types of systems include servomechanism control of an electrical motor with a position feedback device mounted on the shaft; a machine activated by a hydraulic piston with a position feedback device; control of the electrical current in an electrical motor; control of the water level in a boiler; control of aeronautical equipment such as aircraft wing flap position systems, valve control in chemical process plants, etc.
The inventive method is applicable in all of these types of systems to reduce the tracking error, i.e., the difference between the desired state and the actual state of the controlled variable. It also increases the speed of response, reducing the time necessary for the system to reach a new state value within a given precision. Further, it increases control tightness, i.e., it reduces the tracking error induced by external perturbations.
A feature of the inventive method is its ability to improve the performance of servomechanism control systems which utilize a digital feedback sensor. Unlike with prior art systems, which have limited gain when using a digital feedback sensor, the inventive method enables the gain to be varied, by raising it only when necessary, such as when a perturbation occurs, or when the the speed is increased.
For example, in prior art systems, such as a position control system using an electrical motor with an optical encoder, the discrete nature of the encoder reduces the maximum loop gain of the system at very low speeds. At low speeds, the speed sensing based on the encoder is not accurate, and raising the loop gain produces strong vibrations around the equilibrium position. The inventive method allows for a variable gain, thus enabling tighter control.
Other features and advantages of the invention will become apparent from the following drawings and the description.